In the mining environment, explosive charges are used to remove rock and other ground material as part of normal mining operations. Explosive emulsions are often used to provide the explosive energy. Such emulsions are fluid or viscous cohesive matter which can be sprayed into the locations where it will be used.
A typical arrangement for the purposes of explosive removal is to have an underground, substantially horizontal passageway (drive), with downwardly extending hollow shafts or “blast holes”. Such blast holes are often arranged to fan outwardly in a downward or upward direction from the drives in a series of blast holes transversely from the direction of the drive. Accordingly, such formations are known as “fanned rings”. Indeed, there are usually a number of adjacent groups of fanned rings, with their openings into the floor of the passageway being arranged in rows.
It is important to carefully control the energy of the explosive forces used, to effect perimeter and brow control of blasting in mining production areas and to effect suitable explosive energy distribution in fanned rings. In particular, it is important to minimize undesirable damage to surrounding rock and earth matter, and thereby delimit the zone in which the explosive energy will remove such rock and other matter during blasting.
Also, given the fan-like arrangement of the rings, it will be appreciated that the rings are closer to one another at their upper ends than at their lower ends. Accordingly, the explosive forces required to remove the intermediate rock and other matter needs to be less towards the upper ends of the rings given the compounded effect of the explosions in the multiplicity of adjacent rings.
One method of achieving this is to reduce the density of the explosive charges in an upward direction, towards the upper ends of the rings. However explosive emulsions are not particularly practical for such use for reasons mentioned below.
The density of the emulsion can be reduced by an aerating process and this reduction in density can be used to lower the explosive energy capabilities towards the upper ends of the rings. However, in order for the explosive emulsions to be effective, the minimum desirable density is 0.8 g/cm3.
To compensate for the minimum effective density of the emulsions, in addition to reducing the density, the overall compounded explosive energy can also be reduced by providing the explosive emulsions up to different heights in the respective rings, to form a staggered arrangement. However, this process of inserting such emulsions is cumbersome and time-consuming and not particularly effective.
Another problem of the known method of spraying explosive emulsion in blast holes is that the overall weight of the emulsion in the blast hole is such that, as a result of hydrostatic pressures and the reology of the emulsion, only a limited column height of the emulsion (usually 35 m to 45 m depending on whether the emulsion is applied in an up-hole or down-hole direction) can be supported.
Another available method of reducing explosive energy is to use so-called decoupled explosive products. In the above-mentioned environment, this might take the form of explosive materials or emulsions which do not fully fill the blast holes, but which have open spaces between the explosive material and the surfaces (inner walls) of the holes. The open spaces allow dissipation of the explosive energy.
U.S. Pat. No. 5,584,222 (Engsbraten et al) discloses a method for charging explosives in substantially horizontal boreholes. The method comprises the steps of introducing a charging hose with an end opening into at least one substantially horizontal borehole of a blasting round. A pumpable and coherent bulk explosive is then pumped through the charging hose at a controlled rate, and simultaneous with the pumping of the explosive, the hose is withdrawn at a controlled rate. In this way, the pumping and withdrawal rates are adjusted to form a coherent string exiting from the hose end opening. The exiting string only partially fills up the borehole diameter. This can best be described as horizontal string charging.
In other words, the exiting string of explosive has a diameter that is considerably smaller than the diameter of the substantially horizontal bore hole. As the borehole is substantially horizontal, under the influence of gravity, the exiting string sits on only the lower surface of the borehole. It can be appreciated that such a system would therefore not be particularly suitable for substantially vertical bore holes, as the overall weight of the emulsion and as a result of hydrostatic pressures, the emulsion would not stick to the side walls of the vertical boreholes.
One previous attempt at providing a method and apparatus for charging boreholes with explosives, which is suitable for use with substantially vertical boreholes, is described in U.S. Pat. No. 6,397,754 (Perlid). This document discloses a method and a device for charging boreholes with explosives. One end of the charging hose is introduced to a substantially predetermined distance from the bottom of the borehole. Subsequently, a pumpable explosive is pumped through the charging hose at a controlled rate and substantially simultaneously with the pumping of the explosive the charging hose is withdrawn from the borehole at a controlled rate.
Unlike the method described in U.S. Pat. No. 5,584,222, in U.S. Pat. No. 6,397,754, the explosive is caused to flow out from a nozzle, arranged on the end of the charging hose, in the form of a hollow cone and at high pressure, so that the outflowing explosive is given increased viscosity and by virtue of the high outflow rate cohesively adheres to the entire cylinder-shaped wall portion of the borehole, upon which the explosive impinges in connection with the outflowing. With the aid of a centering device, the nozzle is centred in the borehole.
In other words, the nozzle is adapted to spray the explosive in the form of a hollow cone towards the cylinder-shaped wall of the borehole, where the explosive is deposited as a ring in the borehole. This can best be described as vertical, cylindrical decoupled charging.
It is an object of the present invention to provide an explosive charging, and in particular a method of charging that overcomes or ameliorates the disadvantages of the prior art, or at least provides a useful alternative thereto.